Seven years after launch on a four-planet gravitational bank shot covering more than 2 billion miles, NASA's $3.3 billion nuclear-powered Cassini probe - the most sophisticated robotic spacecraft ever built - has finally reached the solar system's most spectacular target: The ringed planet Saturn.

An artist's concept shows the Cassini space probe at Saturn. Credit: NASA/JPL

Kicking off a four-year orbital study of nature's own "lord of the rings," Cassini flew past the strange moon Phoebe June 11, passing within 1,285 miles of the tiny satellite. Seen as a mere speck of light from Earth and a blurry orb from the Voyager 2 probe in 1981, Phoebe was revealed as an irregular, heavily cratered world that defies easy explanation.

The Phoebe flyby was a priceless opportunity to study what may be a captured chunk of debris left over from the birth of the solar system. But it was just the opening act in one of the most scientifically rich voyages of planetary exploration ever attempted.

"It is really, truly the flagship mission of our time," said Carolyn Porco, chief of Cassini's imaging team and an authority on Saturn's rings. "We are exploring the richest planetary system available to us with this magnificent spacecraft, which is the most sophisticated suite of instrumentation ever taken into the outer solar system. So we are going to discover many things.

"And on top of all of that, Saturn is the most alluring of all the planets, it's the icon among planets and that's where we're going. ... I think that people, from the email I'm getting from the public, are very jazzed about this mission. They do very much have the feeling that they are stowaways on this spacecraft."

Animation shows Cassini firing its engine to enter orbit around Saturn. Credit: NASA TV/Spaceflight Now

If all goes well, Cassini will brake into orbit around Saturn the night of June 30, firing its main engine for a nerve-wracking 96.4 minutes. Another 51-minute rocket firing in late August will raise the low point of Cassini's orbit and set the stage for a true voyage of discovery.

Equipped with state-of-the-art telescopes, an imaging radar system and a battery of other powerful instruments, Cassini will spend at least four years orbiting the sixth planet from the sun, studying its rings in unprecedented detail, making high-resolution movies of its windy atmosphere, charting its magnetic field and mapping a host of icy moons.

Saturn's largest moon, Titan, will get special treatment in January when the European-built Huygens probe now bolted to Cassini's hull makes a parachute descent to the surface through the moon's smoggy atmosphere.

Bigger than the planet Mercury, Titan is the only moon in the solar system with a thick atmosphere, one in which hydrocarbons fall as rain and liquid ethane pools on its ultra-cold surface.

"Imagine a world that's smaller than Mars and bigger than the planet Mercury, where the air is four times denser at its surface than the air in this room and the surface pressure is about the same as you'd experience at the bottom of a neighborhood swimming pool," said Jonathan Lunine, a University of Arizona physicist and a member of the Cassini science team. "On that world, the distant sun is never seen and at high noon, things are no brighter than a partly moonlit night on the Earth.

"Because of its great distance, the cold is so enormous that water is always frozen out of the atmosphere. Nitrogen is nearly so, but not quite. And the simplest organic molecule, methane, is there to take the place of water as a cloud former, possibly a rain maker and maybe even the stuff of lakes or seas of hydrocarbons.

"The methane is lofted hundreds of miles above the surface of this world," Lunine said before Cassini's launch in 1997. "It's cracked open by sunlight and cosmic rays and a menagerie of more complicated organics is produced from the methane and these then float down to the surface to accumulate over time, perhaps to depths of hundreds of meters or more. Volcanism and impacts shape the surface and provide energy to make ever more complex organic molecules in a planet-wide tapestry that is an organic chemist's dream.

"What I have described to you is Titan, the second largest moon in the solar system, nearly the largest. It was partly revealed to us by Voyager 1 in 1980. Through its many instruments, Voyager discovered and characterized a dense atmosphere around this cold world. Yet ... Voyager's cameras could not penetrate the organic haze and so we still do not know what awaits Cassini-Huygens at the end of its journey."

An artist's concept shows the Huygens craft making its descent to Titan. Credit: ESA

But in the years since Cassini's launch, optical and radar observations from Earth have given scientists at least a hint of what the spacecraft might find. Scientists are convinced lakes or small oceans of liquid hydrocarbons exist on Titan, but not a globe-spanning sea. One way or the other, the European Space Agency's Huygens probe should resolve the matter, perhaps even splashing down in one of the frigid pools.

"Titan is almost certainly not the home of life today," Lunine said. "But the organic chemical cycles that go on may constitute a chemical laboratory for replaying some of the steps that led to life on Earth. Titan is in some ways the closest analogue we have to the Earth's environment before life began and this makes Titan very important."

The Cassini mother ship, meanwhile, will fly through 76 ever-changing orbits of Saturn over the next four years, using the gravity of Titan to warp its trajectory, setting up subsequent encounters. Forty-five flybys of Titan are planned, four of the moon Enceladus, two each for Iapetus, Rhea and Tethys and one each for Mimas, Hyperion and Dione.

Put another way, Cassini will make more than 50 close passes by seven of Saturn's 31 known moons. Flyby altitudes will be as low as 310 miles, but Titan encounters will be limited to about 590 miles to avoid possible aerodynamic effects from flying through the extreme upper reaches of its atmosphere.

Cassini will use its dish antenna to make radar maps of Titan's hidden surface. Special filters will be used to permit its cameras to glimpse the surface through specific spectral "windows." While scientists aren't sure what they will see, the mission could provide key insights into how life began on Earth.

"Probably the most important thing that our generation can do is to understand the evolution of life in our solar system and throughout the universe," said Charles Elachi, director of the Jet Propulsion Laboratory and leader of the Cassini radar experiment. "In a sense, Cassini will write one of those chapters ... in the book of how life evolved in our universe."

And throughout the voyage, Cassini's instruments will study Saturn itself, its rings and its space environment.

Workers put final touches on Cassini at Kennedy Space Center prior to launch in 1997. Credit: NASA-KSC

"This mission's objective is a four-year, close-up study of the Saturnian system, including its atmosphere, the magnetosphere surrounding Saturn, those rings, its many icy moons and the large moon, Titan," said Wesley Huntress, NASA's associate administrator for space science at the time of Cassini's launch. "The mission represents a rare opportunity to gain significant insights into major scientific questions about the creation of the solar system, pre-life conditions here on early Earth and just a host of questions about Saturn itself.

"Saturn, its rings and its moon system hold clues to understanding the origin of our solar system," he continued. "Its rings are a system are not too unlike the early solar nebula out of which our own planets formed. The processes that sustain that ring may yield information on processes of planetary formation and this information in turn can help us understand what happens around other stars as well."

Cassini is the most ambitious - and expensive - interplanetary project ever attempted, eclipsing even the dual Mars Viking orbiters and landers in inflation-adjusted total cost.

Equipped with three nuclear power sources, 12 instruments, multiple radios, a pair of digital data recorders, two primary computers and more than 50 other subcomputers, Cassini is a marvel of late 20th Century engineering.

Standing 22 feet tall and 13 feet wide, the six-ton spacecraft features a 13-foot wide communications and radar antenna, a 40-foot-long magnetometer boom, 22,000 wire connections, 7.5 miles of electrical cabling, 82 radioisotope heater units to keep its internal systems warm, 16 hydrazine thrusters for coarse attitude control, four reaction wheels for finer, gyroscopic attitude control and two main engines (one is a backup) for major course changes.

This graphic highlights some of the key items on Cassini. Credit: NASA/JPL

Virtually every major system and subsystem has redundant backup hardware and an army of computer programmers has developed complex software sequences that will enable Cassini to detect and correct faults on its own, without intervention from Earth. That's not an option when it can take nearly 90 minutes for radio signals, traveling at 186,000 miles per second, to reach flight controllers, informing them of a problem.

"At Saturn, worst case, there's an hour and 30 minutes one-way light time," said Julie Webster, lead spacecraft engineer at the Jet Propulsion Laboratory in Pasadena, Calif. "So by the time we send up a command and get confirmation back, it's often three hours round-trip light time. The spacecraft has to be designed to take care of itself completely for a period of up to two weeks. And that's for normal operations."

Said Lunine: "We've never sent a spacecraft with the kind of instrumentation, the variety and number of instruments and the power of the instruments to the outer solar system prior to Cassini.

"It's really a tour de force," he said in an interview. "And it's going to be, if all goes well, an incredibly exciting four years in the Saturn system. I think we'll learn that this is more than just a solar system within a solar system. There are just things unique about the Saturn system - the rings, Titan, other aspects - that will make it an incredibly interesting place to explore. ... Cassini should really knock our socks off."

MISSION PREVIEW
   PART 1: FLAGSHIP MISSION OF OUR TIME
   PART 2: CRAFT WELL EQUIPPED FOR SCIENCE
   PART 3: OVERCOMING HURDLES AND CHALLENGES
   PART 4: PREVIEW OF SATURN ORBIT INSERTION